The materials of the present invention are most importantly substrate candidates for high temperature polysilicon thin films. Products based on thin films of polycrystalline silicon include solar cells and flat panel displays, in particular active matrix liquid crystal displays. The materials of the present invention are also useful, however, as a substrate material in various electric, electronic, and optoelectronic devices such as, for example, other forms of flat panel displays, solar batteries, photomasks, and optomagnetic disks.
Liquid crystal displays (LCDs) are typically comprised of two flat glass sheets that encapsulate a thin layer of liquid crystal material. An array of transparent thin film electrodes on the glass modulate the light transmission properties of the liquid crystal material, thereby creating the image. By incorporating an active device such as a diode or thin film transistor (TFT) at each pixel, high contrast and response speed can be achieved to produce high resolution displays. Such flat panel displays, commonly referred to as active matrix LCDs (AMLCD), have become the predominant technology for high performance displays such as notebook computers and portable televisions.
At present, most AMLCDs utilize amorphous silicon (a-Si) processes and have a maximum process temperature of 4500C. Nevertheless, it has long been recognized that the use of polycrystalline silicon (poly-Si) would offer certain advantages over a-Si. Poly-Si has a much higher drive current and electron mobility, thereby allowing reduction of TFT size and at the same time increasing the response speed of the pixels. It also is possible, using poly-Si processing, to build the display drive circuitry directly onto the glass substrate. Such integration significantly decreases costs and increases reliability and also allows for smaller packages and lower power consumption. By contrast, a-Si requires discrete driver chips that must be attached to the display periphery using integrated circuit packaging techniques such as tape carrier bonding.
Poly-Si is conventionally made by depositing amorphous silicon onto a glass sheet using chemical vapor deposition (CVD) techniques, and subsequently exposing the coated glass to high temperatures for a period of time which is sufficient to crystallize the a-Si to poly-Si. There are many methods for fabricating poly-Si, which can be grouped in two categories: low-temperature poly-Si methods, which utilize processing temperatures up to about 600.degree. C., and high-temperature poly-Si methods, which typically employ temperatures as high as 900.degree. C.
Many of the low-temperature methods employ techniques such as laser recrystallization, in which the substrate is held at a temperature of 400.degree. C. and an excimer laser is used to melt and recrystallize the Si layer. The main disadvantage of laser recrystallization is difficulty in achieving good uniformity across the sample. Most of the TFTs have more than sufficient mobilities for on-board logic, but the fact that only a small area can be melted and recrystallized at a time leads to stitching problems. Low temperature poly-Si TFTs can also be made by thermally crystallizing amorphous silicon (maximum temperatures of 600.degree. C.), but in order to make high quality transistors at such low temperatures the films typically must be treated for at least 25 hours. In addition, there are commonly several other high temperature processes following the crystallization step, including growing or annealing of the gate oxide and dopant activation.
The highest quality poly-Si TFTs are fabricated at temperatures of at least 900.degree. C.: such processes enable formation of poly-Si films having extremely high electron mobility (for rapid switching) and excellent TFT uniformity across large areas. This fabrication process typically consists of successive deposition and patterning of thin films using elevated temperature processes which result in the substrate being heated to temperatures in the range of 900.degree. C. There are very few materials capable of meeting this requirement. One approach has been using fused or vitreous silica as the substrate. Fused silica has a sufficiently high strain point of 990.degree.-1000.degree. C. but its thermal expansion coefficient (C.T.E.) is significantly lower than that of silicon, however, with a C.T.E. of 5.times.10.sup.-7 /.degree.C. versus silicon's 37.times.10.sup.-7 /.degree. C. Furthermore, fused silica substrates are extremely expensive to produce, to the point where using them in large display applications is cost prohibitive.
It would therefore be desirable to develop transparent glass-ceramic materials having high strain points and coefficients of thermal expansion which are compatible with polycrystalline silicon, especially high temperature poly-Si.